The present invention relates to an ultrasound diagnostic apparatus, particularly to an ultrasound diagnostic apparatus, a sound velocity setting method, and a recording medium for setting sound velocities of ultrasonic waves in a subject.
Ultrasound diagnostic apparatuses using ultrasound images are put to practical use in the medical field.
In general, this type of ultrasound diagnostic apparatus includes an ultrasound probe (hereinafter also called “probe”) having a piezoelectric element array in which piezoelectric elements transmitting and receiving ultrasonic waves are arranged, and a diagnostic apparatus body.
The ultrasound diagnostic apparatus transmits ultrasonic waves from the probe into a subject's body, receives the ultrasonic echo from the subject with the probe, and electrically processes the resulting reception signals with the diagnostic apparatus body to produce an ultrasound image.
The piezoelectric element array of the ultrasound probe receives through a plurality of piezoelectric elements an ultrasonic echo resulted from one transmission of an ultrasonic beam. Accordingly, even though an ultrasonic echo results from reflection at the same reflection point, the time taken to enter each piezoelectric element varies depending on the position of the piezoelectric element.
To cope with it, the ultrasound diagnostic apparatus performs delay correction separately on reception signals output from the ultrasound probe using a delay time corresponding to, for example, the position of each piezoelectric element, and performs matching addition with phases having been adjusted by the delay correction to produce a sound ray signal (sound ray data), thereby producing a proper ultrasound image without distortion.
The delay correction is performed with the use of the sound velocity of ultrasonic waves (hereinafter also referred to as simply “sound velocity”) in the subject. In a conventional ultrasound diagnostic apparatus, the sound velocity in a subject is assumed to be constant, so that the sound velocity of ultrasonic waves is fixed to a certain set value (e.g., 1520 m/sec).
However, the sound velocity of ultrasonic waves in a subject is not constant since the sound velocity varies depending on differences in tissue property in a fat layer, a muscular layer, and the like in a living body. Specifically, the sound velocity of ultrasonic waves in a subject differs depending on the position. In addition, a fat examinee and a thin examinee differ in the thickness of a fat layer or a muscular layer.
As a result, in the case of using such a conventional ultrasound diagnostic apparatus in which the sound velocity of ultrasonic waves is fixed, an actual sound velocity in a subject and a set sound velocity are often different from each other.
When the set sound velocity is different from the actual sound velocity, it hinders accurate delay correction. Consequently, a produced ultrasound image is to, for instance, contain a distortion as compared to the actual subject, so that the image quality of the ultrasound image is degraded.
To cope with it, JP 2011-92686 A proposes an ultrasound diagnostic apparatus that sets regions of interest by dividing the inside of a subject (ultrasound image) into multiple regions, and sets a sound velocity for each of the regions of interest. Specifically, in this apparatus, transmission and reception of ultrasonic waves for forming a transmission focus point corresponding to a region of interest is performed. Reception signals obtained through this transmission and reception are set with a plurality of sound velocities and delay correction and matching addition are performed to calculate a focus index (e.g., luminance) for every region of interest at each sound velocity. From the result, the sound velocity with which the highest focus index has been obtained is set as the sound velocity of the region of interest.
According to JP 2011-92686 A, the ultrasound diagnostic apparatus can deal with the individual variability of subjects or differences among regions in a subject, which enables to set accurate sound velocities to perform delay correction. Therefore, the ultrasound diagnostic apparatus can produce a high quality ultrasound image without distortion or the like.
Furthermore, in the ultrasound diagnostic apparatus stated in JP 2011-92686 A, the inside of a subject is finely divided by, for example, defining regions of interest to be small and a sound velocity is set for each local region, whereby an ultrasound image with still higher image quality can be produced.